As viewed by a hydrophone, the signal from normal targets is not constant in amplitude or number of echo signals received. A measure of relief from this situation may be obtained by the use of amplitude threshold devices to exclude relatively weak signals not characteristic of expected intruder targets. However, this approach also tends to reduce the effective range of target surveillance in that at greater distance intruder targets also may produce weak signals and thus be lost to detection by the system.
Other means previously employed to provide reliable maximum range detection include systems based upon the doppler principle to differentiate between moving and stationary targets. The doppler effect means is limited in its overall performance by the fact that at some target radial velocity its doppler shift in frequency is not sufficient to allow it to be separated from the echo return from stationary targets.